A conventional transducer system of this type, e.g. as marketed under the designation U89 by the assignee of my present invention, comprises a capacitive microphone of high input impedance virtually constituting an open circuit for direct current, this microphone is connected to an amplifier which feeds an alternating-current load through a step-down transformer and a two-wire line. Biasing voltage for the microphone and operating current for the amplifier are supplied by a d-c source whose high-voltage terminal is connected to the two line conductors via a pair of symmetrical resistors forming part of a phantom circuit. A convenient d-c source is a 48 V battery which, in the particular system referred to, lets the amplifier operate with an output power of about 1 mW RMS; with a load--e.g. a measuring instrument or a speaker--having an impedance of 1 kilo-ohm, this corresponds to a current of 1 mA for an output voltage of 1 V RMS. With the supply voltage referred to, delivered to the line conductors via resistors of 6.8 ohms each, the maximum output voltage of the amplifier is 10 V RMS which calls for a step-down ratio of 1:10 of the coupling transformer.
The interposition of such a transformer between the amplifier output and the load tends to distort the signal and also encumbers the assembly. Prior attempts to eliminate this transformer have led to imperfect impedance matching with resulting limitation of the dynamic range of the microphone.